the water can act, and into which they can penetrate these substances, coming to take fire, will generate a large quantity of air, whose spring being compressed in little room, as in that of a cavern, will not only shake, the superior ground, but seek for passages to escape, and to extricate itself. The passages that present themselves, are the caverns and canals formed by the subterraneous waters and rivulets; the rarefied air will violently precipitate into all the passages that are thus open, and form a furious wind in these subterraneous cavities, the noise of which shall be heard at the surface of the earth, accompanied with concussions. This subterraneous wind, produced by the fire, will extend as far as these cavities, and cause an earthquake more or less violent, in proportion as it is near or remote from the fire, and finds passages more or less narrow: this motion being performed lengthwise, the concussion will be in the same direction, and the earthquake be felt in a straight tract of ground. This air will cause no eruption or volcano, because it will find sufficient room to dilate itself, or rather, because it will find out-lets, and issue out in the form of wind or vapour : or even should it be denied, that there are actually any subterraneous passages by which

this air, and these vapours can escape, it is easily to be conceived, that in the very place where the first explosion happens, the ground being raised to a considerable height, the ad-. joining place may be divided and rent asunder horizontally, in order to correspond with the primary motion, which is sufficient to make passages that may communicate the motion to a very great distance.

Ingenious as these ideas must be allowed, and true as they unquestionably are, in local and confined situations, it yet is with abundance of probability, I might even say of certainty, to be contended, that another cause, still more powerful and universal, is to be given to extended concussions of the earth. Fire, or rather vapour proceeding from the operation of fire, we have already seen capable of dreadful commotions. But the more subtile agency of the electrical fluid, has not yet been adverted to. Descartes boldly pronounced earthquakes to be shocks of electricity, and to be similar to those of thunder. The idea was great, and from experiment and analogy, seems to be confirmed. Yet, as I said before, we must not adopt it indiscriminately. To elementary fire and vapour we must ascribe what belongs to them. Electricity

tricity then will come in for its appropriate and larger share of consideration. Nor will we disunite them altogether, for there are instances, in which they operate conjointly.

But

Braccini observed in 1631, that a column of smoke from Vesuvius, extended over several miles of the country, from which deadly lightning proceeded. The same happened in 1767, when the iron rods erected in Naples became electric whenever Vesuvius emitted fire. Dr. Stukely was the first in our country, who strenuously insisted upon it, that earthquakes are solely occasioned by electricity. In his very learned investigation of this subject, he will not allow they can be occasioned by subterraneous winds, by fires, by vapours, or by any thing that occasions an explosion, and heaves up the ground; for the operation of such a fermentation, says he, would be many days in continuance, and not instantaneous, and the evaporation of the inflammable matter would require a long space of time. Moreover, the whole system of springs and fountains would be ruined, (contrary to fact), wherever earthquakes prevail. Ships at sea are struck, as if they thumped their bottoms; even fishes are affected. The stroke must, therefore, be occasioned by something

VOL. II.

I

thing that can communicate motion with unspeakably greater velocity than any heaving of the earth under the sea from the elasticity of generated vapours. Such could only produce a gradual swell, and could never give such an impulse to water, as would make it feel like a stone. An earthquake is preceded by a warmth, and dryness of air, essentially necessary to electricity; by the Aurora Borealis, by fire balls, thunder, lightning, and coruscations.

The venerable Beccaria, still corroborated this hypothesis, and went so far as to say, that powerful lightning had not unfrequently occasioned, or at least been attended by shakings of the earth. But, in reality, to what other less powerful cause can we attribute the universal and almost momentary phænomena of earthquakes? The rapidity of fire, of the percussion of air, and of vapour, we clearly understand. But can the following facts be attributed to either of these? In the reign of Valentinian, there was an earthquake by which the whole known world was shaken. On the 1st of November, 1755, when the memorable earthquake happened at Lisbon, the seas, lakes, ponds, and rivers, were affected all over Europe: and strange explosions were heard by some of the

workmen

And

workmen in the mines in Derbyshire. yet at the same time, and it is a circumstance very extraordinary, that burning mountains have been observed to keep time with one another in very remote parts of the world. Gassendus says, there happened an eruption at the mountain Semo, in Ethiopia, at the same time with that of Vesuvius in 1633. The same has been observed of Ætna, Vesuvius, and Stromboli; but they being near, it is not so remarkable that they should act in concert: whereas Semo is 2000 miles from Vesuvius. On the day so

fatal to Lisbon, the island of Maderia was violently shook by an earthquake, attended by subterraneous explosions; and about an hour and a half after the tremor had ceased, the sea, which was quite calm, retired suddenly, and then rushed in fifteen feet above high water mark. At Lisbon itself, the sea rushed towards the city like a torrent, though against wind and tide, and rose forty feet higher than ever was known. At Cadiz, on the same day, the sea appeared sixty feet higher than common; beat in the breast-work of the walls, and carried guns of eight and ten tons weight, forty or fifty yards from the wall. At Tangier, Arzila, Salle, and Saffe, the same phænoI 2

* Philosophy of the Elements,

mena